Low-alloyed high strength steel having resistance to the sulfide corrosion cracking

ABSTRACT

A LOW-ALLOYED HIGH STRENGTH STEEL HAVING A LOW SUSCEPTIBILITY TO SULFIDE CORROSION CRACKING CONTAINING   PRECENT C 0.10-0.20 SI 0.10-0.50 MN 0.20-1.00 CR 0.50-2.00 MO, LESS THAN 0.60 V, TI OR CB, LESS THAN 0.10 AL, LESS THAN 0.15 EITHER W AND/OR TA, 0.02-1.00   AND THE BALANCE BEING FE AND IMPURITIES.

Aug. 17, 1971 KATSUYA INOUYE ET AL 3,60

LOW-ALLOYED HIGH STRENGTH STEEL HAVING RESISTANCE TO THE SULFIDECORROSION CRACKING Filed July 5, 1966 I IV l/E/V 70/?5 Kafsuya [nouye Hideya O/rado Yuzo Haso/ Ken/chi Yu/rawa B);

United States Patent 3,600,161 LOW-ALLOYED HIGH STRENGTH STEEL HAVINGRESISTANCE TO THE SULFIDE CORROSION CRACKING Katsuya Inouye, HideyaOkada, and Yuzo Hoso Tokyo, and Kenichi Yukawa, Kawasaki, Japan,assrgnors to Nippon Steel Corporation, Tokyo, Japan Filed July 5, 1966,Ser. No. 562,870 Claims priority, application Japan, July 9, 1965,

0/ 41,118 Int. Cl. C22c 39/14 US. Cl. 75-126F 1 Claim ABSTRACT OF THEDISCLOSURE A low-alloyed high strength steel having a low susceptibilityto sulfide corrosion cracking containing Percent C 0.10-0.20 Si0.10-0.50 Mn 0.20-1.00 Cr 0.50-2.00 Mo, less than 0.60 V, Ti or Cb, lessthan 0.10 Al, less than 0.15 Either W and/or Ta, 0.02-1.00

and the balance being Fe and impurities.

This invention relates to a low-alloyed high strength steel having a lowsusceptibility to the sulfide corrosion cracking.

In general, the addition of nickel increases most remarkably thesusceptibility of steel to the sulfide corrosion cracking. For example,it is well known that the T-lA steel containing no nickel, developed bythe United States Steel Co. has low susceptibility to theabove-mentioned corrosion cracking as compared with the T-l steelcontaining Ni by the same company and this is true also about recentlydeveloped Ni free, lowalloyed, high strength steels. However, theseconventional steels are yet insufiicient in regard to the susceptibilityto the corrosion cracking.

Therefore, an object of this invention is to provide Ni free highstrength steel having an yield strength of at least 70 kg./sq. mm. and atensile strength of about 90 kg./sq. mm. in a quenched and temperedstate and having a low susceptibility to the sulfide corrosion cracking.

Other object of this invention is to provide high strength steel havinga favorable toughness at low temperatures similar to or higher than thatof conventional Ni free high strength steel of an 80 kg./sq. mm. gradein tensile strength and at the same time having a low susceptibility tothe sulfide corrosion cracking.

Other objects of this invention will become apparent from the followingexplanations referring to the accompanying drawings, in which:

FIG. 1 is a sectional view of a test piece subjected to the test formeasuring the susceptibility to the corrosion cracking;

FIG. 2 is a schematic plane view of the test piece shown in FIG. 1 andFIG. 3 is an enlarged view showing a part of the test piece shown inFIG. 1.

The numerals shown in each figure indicate the dimensions of each partof the test piece and the unit is millimeter.

7 The sulfide corrosion crackingof high strength steel has beenconsidered to be caused by hydrogen embrittlement. The inventors haveconfirmed, as the results of the investigations for many years about therelation between the sulfide corrosion cracking and hydrogenembrittlement, that the sulfide corrosion cracking occurs when more thana definite amount of diffusible hydrogen is absorbed in steel. Thehydrogen absorbed in steel is generated by the reaction between hydrogensulfide and steel, and hence the corrosion property of steel in hydrogensulfide atmosphere plays an important role in the matter. Accordingly,the inventors have extensively investigated the influences of thealloying elements on the electrochemical characteristics of steel in anaqueous hydrogen sulfide and as the results thereof it has been foundthat the addition of a small amount of alloying elements gives almost noinfluences on the anodic polarization characteristics, but the cathodicpolarization characteristics are considerably influenced by the kind ofthe alloying element to be added. Further, it has also been found thatthere is a correlation between the cathodic overvoltage of steel and thesusceptibility to cracking, that is, if the cathodic overvoltage ofsteel is high and is not varied as time passes. the cracking of steeldoes not tend to occur. In other words, by adding the alloying elementscapable of increasing the cathodic overvoltage of steel in an aqueoushydrogen sulfide, the occurrence of the sulfide corrosion cracking willbe prevented. On the other hand, it has been confirmed theoretically aswell as experimentally that the fracture stress of steel is higher asthe crystal grain is finer.

Taking into consideration of these points, the inventors haveinvestigated the effect of various elements on the sulfide corrosioncracking of steel, and have found that tungsten and tantalum are theeffective components for satisfying the properties to the above-statedcorrosion cracking of steel and that the presence of nickel gives badinfluences on thus improved properties of steel by the addition oftungsten and/or tantalum.

That is, the feature of this invention is to reduce the susceptibilityof steel to the sulfide corrosion cracking by adding to nickel free highstrength steel tungsten and/or tantalum which is selected as an additivefor satisfying the various factors as the addition elements for steel.Other feature of this invention is that molybdenum is also an elementfor increasing the susceptibility to cracking, but in the case of addingtungsten, the proportion of molybdenum can be reduced as low as pos-'sible, and hence by the addition of tungsten, the susceptibility tocracking can be reduced and at the same time the amount of expensive Mocan be saved.

The composition range of the high strength steel having a lowsusceptibility of the sulfide corrosion cracking is as follows: 0

The reason for the limitation in the composition range of the highstrength steel of this invention is as follows:

The content of carbon is defined to be less than 0.2% by weight in orderto increase the toughness of steel as well as to improve the weldabilitythereof.

Silicon is added in the amount required in the production of steel.

It is desirable to reduce the amount of manganese as low as possible inorder to reduce the susceptibility to the sulfide corrosion cracking butis defined to be 0.2- 1.0%, since at least such an amount of manganeseis necessary for maintaining hardenability of steel and for easy steelmaking.

Chormium is added to increase the hardenability of steel, and at thesame time, to increase the strength as well as to improve the corrosionresistance to sulfides, but if the content of chromium is higher than2.0%, the weldability of the steel is reduced and if less than 0.5% theabove purpose will not be satisfied.

Molybdenum is required to be added less than 0.6% for increasing thehardenability, preventing the occurrence of temper embrittlement, andincreasing the resistance to tempering. However, from the viewpoint ofreducing the susceptibility to the sulfide corrosion cracking, the lessamount is desirable, and as mentioned above, in the case of addingtungsten, the content of molybdenum may be decreased, or even omitted ifdesired.

Copper is an effective element for improving the corrosion resistance ofsteel to sulfide and to the atmospheric corrosion and the necessaryamount is less than 0.5%, preferably 0.2 to 0.3%. However, Cu may beomitted as the case may be.

Vanadium contributes to increase the resistance to tempering, andtitanium and columbium contribute to increase the resistance totempering and to fine the crystal grain of the steel. They may be addedalone or as a mixture thereof but for preventing the reduction oftoughness of steel at low temperatures, the content of them should beless than 0.1%. Moreover, vanadium, titanium or columbium isparticularly effective for the increase of tempering resistance in thecase where molybdenum does not coexist or its amount is very small.

Boron is incorporated in an amount of less than 0.005% for increasingthe hardenability but in the case of thin plate, boron is unnecessary.

Tungsten and/or tantalum is necessary for reducing the susceptibility tothe corrosion cracking but if the content is above 1% the weldabilityand the toughness at low temperatures are reduced and if the content islower than position, the steel may be subjected to conventional heattreatment, quenching and tempering. Thus, while a specific heattreatment has already been proposed to prevent the occurrence ofcorrosion cracking by hydrogen sulfide, but according to the presentinvention, the steel may be subjected to usual quenching and temperingto obtain the above-mentioned purposes. That is, the steel is heated ata temperature above the A transformation point for austenitizing,subsequently quenched, but if it is heated to a higher temperature thanit needs, the crystal grain will be coarsened and the notch toughnesswill be reduced. Hence, the suitable heat-treating temperature is fromthe A transformation point to 950 C. The proper tempering temperatureafter quenching is from 600 to 700 C.

Further, there are no specific limitations about the furnace forsmelting the steel of this invention, and any conventional furnace, suchas, a converter, an open-hearth furnace, or an electric furnace may beemployed.

The following examples are intended to illustrate our invention but notto limit the invention in any way.

EXAMPLE 1 The steel pieces having the chemical compositions shown inTable 1 were water-quenched from 930 C. and tempered at 600-650 C. Themechanical properties of the results are shown in Table 2.

As a process for testing the susceptibility to the sulfide corrosioncracking, there has usually been adopted an accelerative method whereinthe rupture time of test piece is measured when the test piece issubjected to a definite stress or a definite strain in a 0.5% aceticacid solution saturated with hydrogen sulfide, but in order to conductthe test in the more severe conditions, the method was employed in whichthe rupture time of the test piece having a notch as shown in theaccompanying drawing was measured while subjecting it to a definitestress corresponding to the yield stress of 90% in a 0.5% acetic acidsolution which had been saturated with hydrogen sulfide. The results areshown in Table 3.

In addition, when the same test was conducted in an aqueous saturatedhydrogen sulfide solution instead of employing the above-describedacetic acid solution, no rupture was observed after the immersion of oneweek.

TABLE 1 Reference steel Steel of the present 013 013 013 013 invention013 a. QL'JO QGCnvRCOlQP- 9999999999 Hb- Hbbh- WHWLOMPIWEQH 0 9999999999mmmmmmmwww moooowwcewwh m E 9999999999 1mm ;-MOOM ICO OOuL'M-HQWNQQ:

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0.02%, the purpose of the addition thereof is not accomplished. Theaddition of tungsten and/or tantalum is the fundamental feature of thepresent invention and in particular, tantalum contributes to theincrease of the resistance to tempering and fining of the crystal grainin steel.

Aluminum is added mainly for the purpose of deoxidation and hence thecontent may be less than 0.15%.

In addition, the proportions of molybdenum, vanadium, titanium,columbium, tungsten and tantalum should be determined within theabove-mentioned range while considering the balance with the content ofcarbon, the resistance to tempering the toughness and fining of grainsize.

By the reasons as mentioned above, the chemical components of the steelof this invention are limited to the above-mentioned ranges and foraccomplishing the abovementioned purposes about the steel having theabove com- TABLE 2 2 mm. V-notch charpy KgJmm. kg.-m./em.

Tensile strength TABLE 3 Cracking test (time) s nrated H S+0.5 CHaCOOI-Ilower than that of the above-stated conventional steel. It 20 is evidentfrom these tests that the steel of this invention has a tensile strengthof above 80 kg./sq. mm. and a proper toughness as well as the steel ofthis invention is also excellent in susceptibility to the sulfidecorrosion cracking as compared with that of conventional high strengthsteel of 80- kg./ sq. mm. grade.

Further, the following welding test was applied about the steel of thisinvention shown in Table 1 by No. 4 and No. 8.

That is, a simple layer bead was deposited on the test piece at roomtemperature under the conditions of 175 amperes in arc current, 25 voltsin arc voltages and 152 6 mm./hr. in welding speed. The maximum Vickershardness of the steels of No. 4 and No. 8 were 380-400, which is almostsame as the result obtained about conventional high strength steel ofkg./sq. mm. grade.

Further, the test samples of No. 4 and No. 8 were subjected to themodified Y-shaped slit rupture test (shown in the report Weld CrackingTest of High-Strength Steel by Hiroshi Kihara et al. in Welding Journal,vol. 41; January, 36-48, 1962), in which the test sample was pre-heatedto C. or to C. and then welded under the conditions as shown above. Bythe result, no crack was observed in both samples.

What is claimed is:

1. A low-alloyed high strength steel having low susceptibility tosulfide corrosion crocking comprising 0.10 to 0.20% C, 0.10 to 0.50% Si,0.20 to 1.00% Mn, 0.50 to 2.00% Cr, less than 0.60% Mo, less than 0.10%of at least one of the elements selected from the group consisting of V,Ti and Cb, less than 0.15% Al, 0.03 to 0.5% of Ta, and the balance beingFe and impurities.

References Cited UNITED STATES PATENTS 2,347,375 4/1944 Stargardter75-126C 2,021,781 11/1935 Hildorf 75-126 2,798,805 7/1962 Hodge 75-126C3,044,872 7/ 1967 Hayes 75126C HYLAND BIZOT, Primary Examiner US. Cl.X.R.

